Embodiments of the invention relate generally to alternating current (AC) motors and, more particularly, to an automated system and method for defining operating parameters for a soft starter.
Motor soft starters are devices that control transmission of voltage and current from an AC power source to an induction motor. Soft starters are configured to limit the transient voltages and current inrush to the induction motor during start-up, resulting in a “soft” motor starting. In operation, power from the AC source is passed through switching devices in the soft starter, such as a pair of anti-parallel or back-to-back solid state switches in the form of thyristors or silicon controlled rectifiers (SCRs), to control the current flow and, in turn, the terminal voltages of the induction motor.
In general, the soft starter temporarily reduces torque in a powertrain of the motor during startup via selective control of the thyristors. This reduction allows for reduced stresses on the motor and electrical network, which increases the life of the system. The thyristors are controlled to turn on at a given angle, γ, as measured when the voltage becomes positive. The resulting currents flow through the given phases until they reach zero, at which point the thyristors turn off. This pattern creates a “notch” in the voltage. The larger the notch width, the smaller the rms voltage applied to the motor. Since torque is a function of the square of the rms voltage, the larger the notch width, the smaller the torque. If the notch width is zero, full voltage is applied to the motor. Typically, the soft starting process lasts a few seconds, at the end of which the system reaches its final speed and contactors are closed to bypass the soft starter.
In order to operate typical motor soft starters, the operator must manually enter multiple operating parameters into a user interface to define the turn on angle of the thyristor and resulting notch width profile of the soft starter throughout the soft starting process. These operating parameters include a suitable starting time, a current limit, an initial torque, a torque limit, a notch width, and a kick-start duration and kick-start torque (if applicable). Typically, operating parameters are manually selected based on suggestions in a user manual or based on input from field engineers. Correctly setting the parameters for various motors and loads that result in reliable smooth starts based on this trial-and-error parameter selection can be difficult since machine and load evaluation must be taken into account to determine a correct starting pattern.
Accordingly, it would be desirable to have a system that automatically defines the operating parameters for a soft starter with minimal user input. It would also be desirable for such a soft starter system to recalculate and adjust the operating parameters over time to increase the probability of a motor start and account for load changes over time.